Vaporisation of liquefied gases



y 1961 I J. FORREST 2,983,107

VAPORISATION OF LIQUEFIED GASES Filed Sept. 29, 1959 Inventor T HN FORREST ttorney United see aw r' f I Patented M 1 1 7 2,983,101 VVAPORISATION F LIQUEFIED GASES lohn Forrest, Cockfosters, England, assignor to The British Oxygen Company Limited, a company of Great Britain 'I'hisiinventionrelates to the vaporisation of liquefied gases of low boiling point and more particularlyto a method ofreducing, pressure surging" in tube type vaporisers for the 'vaporisation anglsuperheating of suchliquefiedgases.

By the term liquefied gases of low boiling point" is meant liquefied gases boiling at 1 atmosphere pressure at temperatures below ambient and particularly below --l50 C. Examples of such liquefied gases are liquid oxygen, liquid nitrogen and liquid methane.

A widely used method of vaporising liquefied gases of low boiling point and superheating the gas so formed to atmospheric temperature consists in passing the liquefied gas through a tubular vaporiser in heat exchange relationship with a warm fluid surrounding the tubes. The tubes are usually mounted with their axes vertical, the liquefied gas being pumped to the lower end of the tubes and gas superheated to atmospheric temperature being withdrawn from the top of the tubes.

It is known that in tubular vaporisers, when the tube diameter is increased beyond a certain limit, the phenomenon known as pressure surging occurs. It is not possible, at economic flow rates to avoid the surging under such conditions. Pressure surging, that is to say ebullition in the vaporiser, is spasmodic and the pump delivery pressure and the temperature of the vaporised gas leaving the vaporiser both fluctuate violently. The phenomenon also produces considerable noise and vibration of the vaporising coil and its connecting pipework. Alternative methods of avoiding pressure surging would be to use an increased number of tubes with reduced diameters, or to raise the pressure of the liquefied gas above the critical pressure of the fluid followed by a reduction in such pressure after evaporation. Both alternatives have obvious disadvantages.

It is an object of the present invention to provide a method of reducing pressure surging in such vaporisers.

It is a further object to provide a vapcriser of this type in which pressure surging does not substantially occur in use.

According to one aspect of the present invention, a method of reducing pressure surging during the vaporisation and superheating of liquefied gases of low boiling point as hereinbefore defined by heat exchange with a fluid whose temperature is substantially higher than that of said liquefied gas in a tube type vaporiser, comprises packing the section of the tube or tubes in which evaporation of the liquefied gas takm place with randomly disposed packing elements.

According to another aspect of the present invention, a tube type vaporiser for the vaporisation and superheating of liquefied gases of low boiling point as hereinbefore defined by heat exchange with a fluid whose temperature is substantially higher than that of said liquefied gas and having a reduced tendency to pressure surging in use has the section thereof in which evaporation of the liquefied gas takes place packed with randomly disposed packing elements.

This increase may be as much as metal.

The packing elements may be any of the well-known elements used for the random packing of gas liquid contact devices, for example, Raschig rings or McMahan saddles. The packing elements are preferably made of Their size will depend on the diameterof the vaporiser tube. Thus for a tube of diameter 2 inches, Raschig'rings of 4 inch diameter are a suitable packing. The density of packing is preferably at least 60% by which is meant that the actual number of rings used are about 60% of the maximum number which it is possible to contain in the tube when randomly packed and tamped down.

The packing promotes turbulence in the .vaporising section of the vaporiser and reduces spasmodic ebullition. Fluctuations of the pump delivery pressure and the temperature of the gas leaving the vaporiser are substantially reduced and there is a considerable reduction in noise and vibration from the tube or tubes and the associated piping. The flow through the packed vaporiser is very smooth and itsoutputiis appreciably increased.

"The precise length of the tube which requires to be packed in accordance with the present invention will depend on the diameter of the tube, the pressure and throughput of liquefied gas and the particular liquefied gas being vapon'sed. Thus, for liquid oxygen at a pressure of 30 p.s.i.g. and using steam at 40 p.i.s.g. for heating, then the surface area of the packed section will be approximately 0.53 sq. ft. per 10,000 s.ft. per hour of oxygen evaporated. For liquid nitrogen and liquid methane, the corresponding figures are 0.43 and 0.68 respectively.

It is essential in packing the vaporiser in accordance With the present invention that the evaporating section should be substantially completely packed, since if only a part of this section is packed, pressure surging will still take place. To illustrate this, experiments were made with a single tube vaporiser for liquid oxygen under conditions such that the length of'the evaporating section was between 4 and 6 feet. The packing was /1 inch Raschig rings. With a packed section of length 4 ft. at 70% packing density and a pump suction pressure of 9v p.s.i.g., the pump delivered pressure was 40 p.s.i.g. fluctuating i5 p.s.i.g. with peaks of 50 p.s.i.g. The term perature of gaseous oxygen leaving the vaporiser was 20 C. :20 C. and was diflicult to control. The output of the vaporiser was 59,000 s.ft. /hr. of oxygen.

With the vaporiser packed for a length of 6 ft. at 63% packing density, the pump delivery pressure was steady at 40 p.s.i.g. and the temperature of the oxygen leaving the vaporiser was steady at 20 C. The output was 62,500 s.ft. /l1r.

Packing of the vaporiser beyond the evaporating section does not further reduce pressure surging but it increases the pressure drop through the vaporiser and this may cause a substantial drop in throughput.

The invention will now be described with reference to the accompanying drawings in which Figure 1 shows diagrammatically a circuit for the vaporisation of a liquefied gas incorporating a vaporiser according to the present invention; and

Figure 2 shows diagrammatically a part of the vaporiser of Figure 1 in more detail.

Referring to Figure l, the liquefied gas is stored within a' thermally insulated vessel 10 from the bottom of which it passes through a line 11 to a pump 12 where it is pumped to any desired pressure below the critical pressure of the gas. The liquefied gas is then passed by line 13 to an evaporater and superheater consisting of a single tube 14 wound in a helix through which the liquefied gas is passed, enclosed in a vessel 15, through which a heatproviding fluid such as steam is passed by means of an inlet 16 and an outlet 17. In the tube 14, the liquefied gas is first evaporated and the vapour so formed superheated by heat exchange with the heat-providing fluid,

1 the superheated vapour leaving by a line 18.

The tube 14 is shown diagrammatically in greater detail in Figure 2. The liquefied gas enters at a bottom inlet retaining devices 22, 23 in the form of perforated cone- 7 Y shaped metal plates located at each end of the vaporisation section. The tsuperheating section comprises the remainder of the tube 14, the upper retaining device 23 being located at the point along the tube '14 where evaporation of the liquid is complete.

'Iclaimx 1. A method of reducing pressure surging during the vaporisation and superheating of a liquefied gas of low boiling point by heat exchange with a fluid whose temperature is substantially higher than that of said liquefied gas in a tube-type combined vaporiser and'superheater comprising packing only thattsection of the tube or tubes in which evaporation of the liquefied gas takes place to a packing density of at least 60% with randomly-disposed, hollow, geometrically-shaped, metal packing elements of the type used for randomly packing gas/liquid contact devices.

2. A tube-type combined vaporiser and superheater for the vaporisation and superheating of liquefied gases of low boiling point by heatiexchange with a fluid whose temperature. is substantially, higher than that of said liquefied gas having only that section of the tube or tubes thereof in which vaporisation of the liquefied gas takes .placepacked to a packing density of at least 60% with randomly disposed, hollow, geometricallyrshaped, metal packing elements of the type used for randomly packing gas/liquid contact devices, whereby pressure rgingisreduced durin a 7 References Gited in the file of this patent UNITED STATES PATENTS 2,565,221 6 Gaug1er Aug. 21, 1951 

